Manholes are critical assets of the nation's underground wastewater infrastructure. Recently updated manhole inspection and rehabilitation guidelines are aimed at improving manhole structural integrity by identifying issues and the appropriate methods of repairing and rehabilitation so that timely maintenance prevents major problems.
Since 1997, the most comprehensive industry guideline for manhole inspections has been the Manual of Inspection and Rehabilitation Practices No. 92, published by the American Society of Civil Engineers (ASCE).
"It was the first manual of practice developed in the United States focusing exclusively on manhole inspection and rehabilitation," says Richard (Rick) Nelson, vice president and director, conveyance infrastructure technology, CH2M Hill. Nelson is the ASCE committee chairman and one of the primary authors of the first edition and a reviewer of the second edition of the manual which was developed by a committee lead by Joanne Hughes, RS Lining Systems.
Nelson says the manual addressed safety, manhole inspections procedures, quantification of inflow and infiltration (I&I), rehabilitation methods, cost effectiveness analysis of rehabilitation methods, and construction inspection and quality control.
"The authors of this first manual included many industry leaders from utilities, vendors and consultants," Nelson says. "The goal of the committee was to publish a manual of practice to help standardize the inspection of manholes and provide guidance on data evaluation, I&I quantification and rehabilitation methods. The manual included color photos of defects as an aid to the practitioner.”
Time for update
That manual has served the industry well. However, it was prepared between 1994 and its final approval and publication three years later, and technologies have changed significantly since then. Nelson provides this summary of key points in the revised guidelines:
A presentation explaining the revised manual was presented last month at the 2010 UCT (Underground Construction Technology International Conference & Exposition) in Tampa and copies of the manual can be ordered on the ASCE web site (www.asce.org [1]) or purchased from outlets that carry ASCE publications. Completion of the manual required about five years of work by many dedicated professionals (see attached sidebar).
Nelson says as of 2007, the estimated number of manholes in the United States was 20 million – significant assets to communities and the primary means of access for sewer collection systems in order to perform maintenance.
"Evaluating and estimating the life of these structures is of considerable importance in the financial planning of agencies particularly in respect to depreciation allowances and rates of return on investment," he says. "Actually estimating the economic life – the age when a manhole becomes more cost effective to replace than to rehabilitate – is a function that must be recognized on a local basis. Therefore, one of the goals of this manual is to provide an inspection and condition grading protocol that provides logical follow up steps which can be taken to maintain and improve upon the health of these structures."
Material evolution
Evolutionary improvements in material for constructing and repairing manholes also make the new guidelines timely.
"Brick commonly used through the 1930s was labor intensive and it required a great number of the bricks to build the structure," Nelson says. "Concrete materials were a significant evolution as the manhole could be built in lifts or segments, utilizing precast materials. Today, another factor driving change results from attacks of nature in the forms of water infiltration and hydrogen sulfide (H2S). H2S issues have driven the industry to look for new construction materials which would be infiltration and corrosion resistant as well as allowing for structures to be built in lifts. Manholes are now being created from inert materials, for example fiberglass and polyethylene."
Corrosion severely compromises the structural integrity of both brick and concrete components, costing millions of dollars annually for repairs.H2S is a naturally occurring process that exists in a dissolved state within wastewater.
"When released into a gaseous state, it comes in contact with the moist surfaces of a manhole wall and as concentration levels rise, bacteria colonies proliferate, forming an extremely corrosive slime layer that can rapidly cause weakening and decomposition of even the most massive concrete and steel structures," Nelson says. "Newer materials such as modified cementitious and epoxy coatings have proven themselves to be resistant to this attack while maintaining their integrity."
Excessive I&I is another serious problem for wastewater collection and treatment systems, and Nelson points out that the hydraulic effects of these extraneous flows are particularly important because they utilize valuable collection and treatment system capacity that is needed for urban growth.
Flows of untreated wastewater from I&I can result in public health economic issues, and environmental issues which are deterrents to the overall objective of protecting the nation's water resources. Prolonged leakage can create voids outside a manhole structure with the potential of removing of soil resulting in the loss of lateral soil support which can create structural issues. The revised manual covers all these and other issues and provides technical guidance for implementing a successful manhole inspection and rehabilitation program.
Nelson cites two essential objectives in managing manhole assets:
"Through an effective manhole inspection program, agencies can accurately identify inventory and evaluate the condition of these structures," he concludes. "An archive trend history will also aid in determining the most cost efficient times for rehabilitation. Methods to remove excessive manhole infiltration and inflow reduce corrosion, improve manhole structural integrity, address public safety related issues and implement general system maintenance needs identified through consistent data gathering techniques.”
Manhole History
The word "manhole" has an interesting history, observes Richard (Rick) Nelson, vice president and director, conveyance infrastructure technology, CH2M Hill.
" ‘Manhole' was first used to describe the access holes between the decks of old sailing ships," he explains. "It wasn't until later that the term was used to describe the structure through which access to sewers for maintenance could be achieved. Perhaps the name was adopted because it was, in essence, a hole into which a person would go to do maintenance, or it was adapted from one level (street level) to another level (the sewer beneath the street).
"And the word ‘sewer' is derived from the term 'seaward' in Old English. Early sewers in the London area were open ditches which led to the Thames River, and from there on to the sea or “seaward'."
Contributors Recognized
The ASCE Manhole Rehabilitation Committee members responsible for the second edition of Manhole Guidelines Manual 92 was composed of: Anthony Almeida, Halff & Associates; James H. Forbes, Pipeline Analysis; Joanne B. Hughes, RS Lining Systems; John F. Jurgens, Trenchless Resources Int.; Larry W. Kiest, LMK Enterprises; Mohammad Najafi, PH.D., PE, University of Texas at Arlington; Richard E. Nelson, CH2M Hill; Lynn Osborn, Insituform Technologies; William E. Shook, AP/M Permaform; John J. Struzziery, SEA Consultants; and Mark G. Wade, CH2M Hill.
Contributing authors, in addition to Forbes, Hughes, Jurgens and Shook, included: Marc Anctil, Logiball; Tim Back, P.E., Back Municipal Consulting; G. Alan Johnson, CH2M Hill; and Stephen Wierzchowski, RLS Solutions.
Providing peer reviews were Nelson, Osborn and Struzziery. Organizations providing cooperation and assistance in preparation of the manual were AP/M Permaform, Avanti International, Cretex Specialty Products, LMK enterprises and the city of Fayetteville, AR.
Links:
[1] http://www.asce.org